元素类型不能分配给小部件

Question

为什么这个错误出现在我的**元素类型“时间轴”不能分配给列表类型小部件**

Scaffold buildAuthScreen() {
return Scaffold(
  body: PageView(
    children: <Widget>[
      Timeline(),
      ActivityFeed(),
      Upload(),
      Search(),
      Profile(),

    ],
  ),
);}

尽管时间轴是小部件

  class Timeline extends StatefulWidget {
  @override
  _TimelineState createState() => _TimelineState();
}

class _TimelineState extends State<Timeline> {
  @override
  Widget build(context) {
    return Text("Timeline");
  }
}